1,609 research outputs found
Constraints on the energy spectrum of non-Hermitian models in open environments
Motivated by recent progress on non-Hermitian topological band theories, we
study the energy spectrum of a generic two-band non-Hermitian Hamiltonian. We
prove rigorously that the complex energy spectrum of such a non-Hermitian
Hamiltonian is restricted to the lower complex plane, provided that the
parameters of the Hamiltonian satisfy a certain constraint. Furthermore, we
consider one specific scenario where such a non-Hermitian Hamiltonian can
arise, namely a two-band model coupled to an environment, and show that this
aforementioned constraint orignates from very general physical considerations.
Our findings are relevant in the definition of the energy gap in non-Hermitian
topological band theories and also have implications on simulations of such
theories using quantum systems.Comment: 7 pages, 1 figur
Estimating the impact of transport efficiency on trade costs: Evidence from Chinese agricultural traders
Using a unique survey data on agricultural traders in China in 2004, this study provides direct evidence on significance of interregional transport costs and their key determinants. Our major findings are as follows: (1) the trade barriers within China are dominated by transport-related costs but not artificial barriers, approximated by tolls and fines; (2) Labor and fuels costs are the most significant component of transport costs; (3) road quality is very important for transportation efficiency. Our results indicate that if increasing transport speed by 1 km per hour now, the fuel costs and total direct transportation costs for Chinese traders would reduce by 1.3% and 0.7% respectively.Transportation Costs, China, Agricultural Traders, Infrastructure, International Relations/Trade,
AN EXPERIMENTAL STUDY OF THE EFFECTS OF ELECTRICAL STIMULATION ON STRENGTH AND FLEXIBILITY
Electrical stimulation (ES) in muscles has widely applied in muscle strength training as a training method. It was proven to greatly enhance muscle strength. The purpose of this study was to examine the changes in muscle flexibility in the training of muscle strength with the use of electrical stimulation. The experiment demonstrated that the use of electrical-stimulation in training (EST) could obtain the effects of the improvement in both of muscle strength and flexibility
Paradoxical constitutive law between donor O-H bond length and its stretching frequency in water dimer
The constitutive laws of hydrogen bonds (H-bonds) are central to
understanding microphysical processes not precisely observed, especially in
terms of structural properties. Previous experiences in water H-bonding
revealed that as the intermolecular O...O distance shortens, the O-H stretching
frequency redshifts; thus, an elongated O-H bond length can be empirically
inferred, which is described as the constitutive law under the cooperative
effect. Here, using the high-precision CCSD(T) method, we report a violation of
the conventional constitutive law in water dimer. That is, when the variation
in the O...O distance changes from stretching by 0.06 to contracting by -0.15
angstrom compared to the equilibrium position, the donor O-H bond length
decreases from 0.9724 to 0.9717 angstrom, and the O-H stretching frequency
redshifts from 3715 to 3708 cm-1. Our work highlights that the O-H bond length
decreases simultaneously with its stretching frequency, which is clearly
inconsistent with the previously recognized constitutive law.Comment: 10 pages, 2 figure
Experiments and simulations of MEMS thermal sensors for wall shear-stress measurements in aerodynamic control applications
MEMS thermal shear-stress sensors exploit heat-transfer effects to measure the shear stress exerted by an air flow on its solid boundary, and have promising applications in aerodynamic control. Classical theory for conventional, macroscale thermal shear-stress sensors states that the rate of heat removed by the flow from the sensor is proportional to the 1/3-power of the shear stress. However, we have observed that this theory is inconsistent with experimental data from MEMS sensors. This paper seeks to develop an understanding of MEMS thermal shear-stress sensors through a study including both experimental and theoretical investigations. We first obtain experimental data that confirm the inadequacy of the classical theory by wind-tunnel testing of prototype MEMS shear-stress sensors with different dimensions and materials. A theoretical analysis is performed to identify that this inadequacy is due to the lack of a thin thermal boundary layer in the fluid flow at the sensor surface, and then a two-dimensional MEMS shear-stress sensor theory is presented. This theory incorporates important heat-transfer effects that are ignored by the classical theory, and consistently explains the experimental data obtained from prototype MEMS sensors. Moreover, the prototype MEMS sensors are studied with three-dimensional simulations, yielding results that quantitatively agree with experimental data. This work demonstrates that classical assumptions made for conventional thermal devices should be carefully examined for miniature MEMS devices
Giant supercurrent states in a superconductor-InAs/GaSb-superconductor junction
Superconductivity in topological materials has attracted a great deal of
interest in both electron physics and material sciences since the theoretical
predictions that Majorana fermions can be realized in topological
superconductors [1-4]. Topological superconductivity could be realized in a
type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a
conventional superconductor. Here we report observations of the proximity
effect induced giant supercurrent states in an InAs/GaSb bilayer system that is
sandwiched between two superconducting tantalum electrodes to form a
superconductor-InAs/GaSb-superconductor junction. Electron transport results
show that the supercurrent states can be preserved in a surprisingly large
temperature-magnetic field (T-H) parameter space. In addition, the evolution of
differential resistance in T and H reveals an interesting superconducting gap
structure
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